1. Field of the Invention
The invention generally relates to electrostatic chucks for supporting a semiconductor wafer in a semiconductor wafer processing system. More particularly, the present invention relates to a process of manufacturing such an electrostatic chuck having relatively small diameter gas inlet channels that supply a heat transfer medium to the surface of the chuck.
2. Description of the Background Art
Typical semiconductor wafer processing apparatus 10 is illustrated in FIG. 1. The apparatus 10 includes an electrostatic chuck 12 having a top wafer support surface 14 in which is embedded an electrode 16 connected to a DC/RF supply 18. The supply 18 supplies DC voltage to the electrode 16 to provide a DC bias voltage to electrostatically retain the semiconductor wafer 20 on the top surface 14 of the electrostatic chuck 12. The supply 18 may also provide RF current or energy to the apparatus 10 to ignite a plasma above the wafer 20. As is known to those skilled in the semiconductor wafer processing art, the use of high plasma power improves etch rates, facilitates increased aspect ratios, and provides various other process improvements. The use of high plasma power imparts additional heat to the semiconductor wafer 20 during processing, and unless heating of the semiconductor wafer 20 is controlled, the semiconductor wafer can be so heated that the partially processed semiconductor wafer becomes damaged. To permit the use of high plasma power, it is known, as illustrated in FIG. 1, to mount the electrostatic chuck on a base 24 which is provided with a plurality of coolant receiving channels 25. The receiving channels 25 carry suitable coolant flows for cooling the electrostatic chuck 12 which in turn cools the semiconductor wafer 20.
In spite of reasonable efforts to make the bottom surface of the wafer 20, referred to in the art as the backside of the wafer, smooth, and to make the top or support surface 14 of the electrostatic chuck 12 smooth, surface irregularities are present that result in interstitial spaces, or spacing, between the backside of the semiconductor wafer 20 and the support surface 14 of the electrostatic chuck 12. This interstitial spacing 26 is indicated diagrammatically in FIG. 2. Vacuum will occupy such interstitial spacing, and a vacuum, as is known, is not a good heat transfer medium for transferring heat from the wafer 20 to the electrostatic chuck 12. It is further known in the semiconductor wafer processing art to enhance the thermal transfer between the semiconductor wafer 20 and the electrostatic chuck 12 by supplying thermal transfer gas, such as helium or argon, to the interstitial spaces 26. The thermal transfer gas enhances the thermal transfer between the semiconductor wafer 20 and the electrostatic chuck 12.
Typically, the thermal transfer gas is supplied, for example, from the helium supply 30 shown in FIG. 1 through a conduit 32 to the interstitial spaces 26 in FIG. 2. Typically, the diameter of the thermal transfer gas conduit 32 is about 0.5-3 mm. It has been found that the diameter of the typical thermal transfer gas conduit, such as conduit 32, is so large that the volume of the thermal transfer gas in the conduit, can become ionized upon the RF energy being applied to the apparatus 10 to ionize the etch plasma 22 (FIG. 1) and such ionization of the thermal transfer gas invites arcing. Such arcing can pit, or otherwise damage, the backside of the wafer 20 to an extent that causes wafer damage. Such arcing can also pit, and otherwise damage, the support surface 14 of the electrostatic chuck 12 to the point where the electrostatic chuck is ruined. As is also known, the ionization of a gas is a function, at least in part, of the volume of the gas, and the larger the volume of the gas, the more likely the gas is to ionize and the smaller the volume of the gas, the less likely it is to ionize. It has been found that the volume of the thermal transfer gas, such as helium contained in the typical prior art conduit, such as conduit 32 shown in FIG. 2, invites or promotes ionization of the thermal transfer gas particularly as higher plasma powers are utilized.
Accordingly, there exists a need in the art for thermal transfer gas inlet channels in an electrostatic chuck that are so small that the volume of thermal transfer gas in such gas inlet channels has a very low tendency to ionize when higher plasma powers are utilized.
The disadvantages associated with the prior art are overcome by an electrostatic chuck and a process of fabricating the electrostatic chuck comprising the step of embedding a plurality of inserts in a ceramic electrostatic chuck, each insert comprising a matrix of the ceramic of which the electrostatic chuck is made and a plurality of removable elongate members, and removing the elongate members to form a plurality of elongate holes that define the plurality of gas inlet channels. The removable elongate members may be etchable wires, a bundle of loosely rolled etchable metal wire mesh or a plurality of etchable lines printed on ceramic and which may be removed by chemical or thermal etching. The result is an electrostatic chuck comprising at least one insert having channels of a diameter that precludes thermal transfer gas ignition.